Electrostatic printing tape



July 14, 1970 D, o' ET AL ELECTROSTATIC PRINTING TAPE Filed 001 2, 1964 G 2. DIELfCTR/C' F/x//v6' MEMBf/i [0567?057 17/6 WM/TING TAPE FIG. 2, 1x F/Xl/VG LAYER l5: CU/ DUCT/VE MEMBER IN VENTOR S DONALD F. O'NEILL BY WALLEY F. ESTES United States Patent 3,520,771 ELECTROSTATIC PRINTING TAPE Donald F. ONeill, Huntingdon Valley, and Walley F.

Estes, Warminster, Pa., assignors to Paper Manufacturers Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Oct. 2, 1964, Ser. No. 401,065 Int. Cl. 1332b 9/04; B44c ]/14 US. Cl. 161-234 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to an improved electrostatic printing tape; and, more particularly, the invention relates to an electrostatic printing tape in which ink particles are fixed in place by pressure alone and which possesses important advantages over prior tapes of this class.

The term tape is used herein for the sake of convenience to denote any sheet product without regard to actual relative length and width dimensions since, while the product may be prepared initially in the form of rolls, it may later be cut into separate sheets.

Prior electrostatic recording tapes have been composed of a conductive layer, like carbon-filled paper, coated with a thermoplastic, dielectric material, like polyethylene. In its use, the tape is passed through a writing head between two sets of closely-separated electrodes. Pulsing the electrodes leaves an electrostatic charge in a precisely defined configuration, on the tape. The tape is then passed through dry, powdered ink which adheres to the surface of the thermoplastic, dielectric layer at the charged areas, excess ink is shaken off and the ink is fixed in the areas to which it adheres by pressing, as by passing between rolls. In this way information can be recorded as a visible pattern on tape at substantially higher speeds and with greater reliability than is the case when the older, mechanical tape-punching process is used. The recorded information may then be read visibly or photoelectrically by virtue of the difference in the reflectance between the inked areas and the non-inked areas. In this connection, the thermoplastic, dielectric layer is normally pigmented to render it opaque and light-colored, usually white, and the ink is contrastingly dark, usually black.

Prior pressure-fixing (as distinguished from heat-fixing) electrostatic tapes possessed limitations. In the first place, polyethylene, although having the requisite dielectric properties for use as the thermoplastic, dielectric material, was found to retain the fixed ink poorly. It appeared that, in time, the polyethylene would release its hold on the imbedded ink particles so that they could easily be removed or dislocated, as by rubbing lightly with a finger, thereby rendering reading diflicult or impossible particularly when done photoelectrically. This was the principal limitation with the polyethylene coated product. Secondarily, it was found that pigmenting the polyethylene to render it light-colored and opaque led to defects in the dielectric properties, with loss of charge, probably due to pin holes in the coating where relatively large agglomerates of pigment are present.

It is the principal object of the present invention to provide a novel electrostatic printing tape of the pressurefixing type.

It is another object of the present invention to provide a novel pressure-fixing, electrostatic printing tape to which the ink particles adhere in a manner superior to that of prior tapes of this class.

It is a further object of the invention to provide a novel pressure-fixing, electrostatic printing tape substantially freer of pinholes, and hence voltage breakdown, than prior tapes of this class.

Other objects will become apparent from a consideration of the following specification and claims.

The novel tape of the present invention comprises a conductive base member and thereover a dielectric and fixing member at least the outer surface layer of which consists essentially of a thermoplastic composition having (1) a yield strength between about 500 and about 800 p.s.i.; (2) a hot melt viscosity of from about 2000 to about 1,000,000 cps. at 240 F.; a flow temperature between about and about 210 R; an elongation of between about 40 and about 400%, and an average molecular weight of between about 10,000 and about 100,000.

The dielectric and fixing member may be in the form of a single, homogeneous layer or, in the preferred embodiment, in the form of discrete layers with the outermost layer being the fixing material having the abovementioned properties and the innermost layer, next to the conductive base member, being the primary dielectric material.

The invention will be more readily understood from a consideration of the drawing in which:

FIG. 1 is a side elevational view in section and greatly enlarged, of a tape according to the broader aspects of the invention, and

FIG. 2 is a side elevational view, in section and greatly enlarged, of a tape according to the preferred embodiment of the invention.

Referring to FIG. 1, this illustrates a tape 1 in which the dielectric-fixing member 2 is a single homogeneous layer, which may be composed essentially of a blend of paraffin wax and a thermoplastic resin.

FIG. 2 illustrates the preferred tape 10 in which the dielectric-fixing member is made up of two distinct layers: a fixing layer 11, which may be composed essentially of a blend of paratfin wax and a thermoplastic resin, and a dielectric layer 12, which may be polyethylene.

Regardless of the embodiment, the conductive base member may be one of the well known resilient conductive sheet materials used heretofore in electrostatic printing tapes. Any resilient sheet material conductive throughout its thickness, like copper or aluminum foil, and paper or plastic films filled with particles of carbon or other conductive material, may be used. Probably the most common material used is carbon-filled paper. In this connection, carbon-filled kraft paper weighing (filled) in the neighborhood of 35 pounds per ream (24" x 36-500 sheets) is particularly satisfactory. The conductive base member should have a relatively low surface resistivity, as between about 5000 and about 400,000 ohms per square inch. The conductive base member may have a thickness in the neighborhood of about 2 to about 4 mils.

The primary contribution of the present invention is a novel and improved fixing member. As stated, the polyethylene layer of prior electrostatic printing tapes would accept initially the ink particles pressed thereinto but would slowly reject these particles due, presumably, to the polyethylenes tendency to return to its original form. Thus it has been found that the fixing layer must not only readily deform or yield under the moderate fixing pressures involved but must also show little elasticity or memory in order to retain the ink particles. It has been determined that, in order to serve satisfactorily in a pressure-fixing, electrostatic printing tape, the fixing layer must have certain basic physical properties: yield strength between about 500 and about 800 p.s.i.; a hot melt viscosity of from about 2000 to about 1,000,000 cps. at 240 F., a fiow temperature between about 160 and about 210 R; an elongation of between about 40 and about 400%, and an average molecular weight of between about 10,000 and about 100,000. Polyethylene has a yield strength of 975 p.s.i. (for the low density type) and 3400 p.s.i. (for the high density type); a hot melt viscosity of 1,000,000 cps. at 240 F.; a flow temperature of about 220 F., and an average molecular weight of from 300,000 to 1,000,000. The fixing layer will also be non-blocking; that is to say, it will not stick to itself under normal ambient temperature conditions, and the foregoing properties should provide non-blocking properties. In addition, the fixing layer will have a relatively high resistance, generally at least 10 ohms per cm.

Compositions possessing the foregoing characteristics can be prepared, for example, by blending paraffin wax and a thermoplastic resin. Suitable thermoplastic resins are copolymers of ethylene and a vinyl compound copolymerizable therewith. By the term vinyl compound is meant a monomer having a vinyl group, such as vinyl acetate, styrene, vinyl chloride, ethyl acrylate, and the like. In such copolymers, the ethylene will predominate making up from about 60 to about 90%, by weight, of ethylene, the balance being essentially the vinyl compound. Illustrative copolymers are: 7l-73% ethylene-27- 29% vinyl acetate; 80% ethylene-20% vinyl acetate; 80% ethylene-20% ethyl acrylate, and 70% ethylene- 30% ethyl acrylate. Copolymers of ethylene and vinyl acetate are preferred.

The parafiin wax blended with the thermoplastic resin is advantageously one having a relatively high melting point, that is from about 140 to about 190 F., and being substantially free of oil, that is less than about 0.25%, by weight, of oil. A preferred paraifin wax has a melting point (D-87) of 183 F.; a melting point (AMP) of 186 F., a viscosity of 75 SSU at 210 F. and an oil content of 0.1%. The preparation of such a wax is disclosed in US. Pat. No. 2,668,140.

In the blend of paraffin wax and thermoplastic resin, the relative proportions may vary, depending upon the particular resin and wax selected, from about 40% of wax and about 60% of resin to about 90% of wax and about 10% of resin. In the preferred formulations the wax makes up between about 60 and about 80% of the blend of the wax and resin.

The wax-resin blend may or may not be pigmented. Reading of the recorded information on the tape is based on contrast between the inked areas and the noninked areas or background. Therefore, the surface to which the ink is applied must be, for example, white, when the ink is black. Where the dielectric-fixing member is a single, homogeneous layer, it is preferred that a pigment be included to provide the desired opacity and contrast. Suitable pigments are white, metal oxides and carbonates, like titanium dioxide, calcium carbonate, and the like. The amount of pigment so employed will be dictated by conventional considerations, and the amount of pigment may range from about to about 25%, by weight, based on the entire pigmented composition. Of course, where the material underlaying the fixing layer is already opaque and of the desired contrasting color, as when the conductive base member is White, or as when, according to the preferred embodiment, the dielectricfixing member is made up of two layers and the dielectric layer is pigmented, there is no need to include pigment in the fixing layer. In this case, the fixing layer will consist essentially of the wax-resin blend. Where reference is made herein to wax-resin or wax-copolymer blends it will be understood to include pigmented as well as unpigmented compositions.

Referring further to the preferred embodiment as illustrated in FIG. 2, the dielectric layer may be any of those materials heretofore employed for this purpose. Essentially it will be a material applicable as a resilient film, as by extrusion or from a hot melt, and which is relatively non-conductive, that is has a volume resistivity of at least about 10 ohms-cm. Examples of materials that might be used are polyethylene, polypropylene, polyvinylidene chloride, polymethylmethacrylate, poylethylacrylate and the like. Polyethylene is presently preferred. Advantageously, in this embodiment the dielectric layer is pigmented, and pigment materials and amounts thereof as discussed above are applicable. A particularly preferred dielectric layer is titanium dioxide-pigmented polyethylene. In this embodiment the dielectric layer may be on the order of 1 mil thick, tnd the wax-resin fixing layer is on the order of 4 mil thick.

The tape of the present invention possesses many important advantages over prior pressure-fixing electrostatic printing tapes. Of primary importance is the high degree of retention of the ink spots. Thus, once the dry, powdered ink has been fixed in place by pressing, there is no tendency for them to be rejected by the material of the fixing layer, and the spots are resistant to attempted wiping or smearing with ones finger. Additionally, the dielectric-fixing member of the present tape is markedly free of pin holes, and this is particularly true in the preferred embodiment where the dielectric-fixing member is made up of two layers. In such case the dielectric breakdown voltage is many times greater than the sum of the breakdown voltages of the individual layers indicating a sealing of pin holes in the dielectric layer by the after-applied fixing layer.

The invention will be more readily understood from the following specific examples which are given for the purpose of illustration only and are not intended to limit the scope of the invention in any Way.

EXAMPLE I This example illustrates the preferred embodiment wherein the dielectric-fixing member is made up of two separate layers.

There is applied, by extrusion, to a conductive 35 lb. (24" x 36"-50O sheet ream) carbon-filled kraft paper a film of TiO pigmented polyethylene (opaque medium density polyethylene). The polyethylene film is approximately 1 mil thick. There is then applied, by gravure roll from a hot melt an approximately mil thick film of a blend of 70 parts of wax and 30 parts of a copolymer of ethylene and vinyl acetate. The wax has a melting point (D87) of 183 F., a melting point (AMP) of 186 F., a viscosity of 75 SSU at 210 F. and an oil content of about 0.1%, and the copolymer is prepared from 71-73 parts, by weight, of ethylene and 27-29% of vinyl acetate. The Wax-copolymer blend has a hot melt viscosity of 3000 cps. at 240 F.; a flow temperature of 182 R; an average molecular weight of 10,000; a yield strength of 725 p.s.i., and an elongation of 59%.

The resulting coated sheet, when cut to appropriate widths for use as a printing tape in a pressure-fixing, electrostatic printer, not only picks up and holds the ink but retains the imbedded ink spots without any apparent rejection of them through any tendency of the waxcopolymer fixing layer to regain its former shape.

The pigmented polyethylene layer alone breaks down at about 600-700 volts presumably because of pin holes at agglomerations of titanium dioxide. The wav-copolymer blend film alone breaks down at about 500 volts. However, the composite layer does not break down until about 4000 volts.

EXAMPLE II This example illustrates the embodiment wherein the dielectric-fixing member is a single, homogeneous layer. A hot melt is prepared at 270-300 F. from 25 parts of ethylene-vinyl acetate copolymer, 20 parts of titanium dioxide and 55 parts of paraffin wax melting at 143- 145 F. This material is applied as a film to the carbonfilled conductive paper of Example 1 to a coating weight of 25*30 lbs. (24" X 36"500). The coating has a whiteness value of 80-82, and the resulting tape performs satisfactorily in pressure-fixing, electrostatic recording equipment.

Modification is possible in selecting the materials employed in preparing the present tape as well as in the techniques employed without departing from the scope of the present invention.

What is claimed is:

1. A pressure-fixing electrostatic printing tape consisting essentially of a conductive base member and thereover and attached thereto a dielectric and fixing member coating made of two layers, the layer next to said conductive base member being a resilient dielectric polyethylene film having a volume resistivity of at least 10 ohms-cm. and the outermost layer consisting essentially of a thermoplastic composition having (1) a yield strength between about 500 and 800 p.s.i.; (2) a hot melt viscosity of from about 2000 to about 1,000,000 cps. at 240 F.; a flow temperature between about 160 and about 210 R; an elongation of between about 40 and about 400%, and an average molecular weight of between about 10,000 and about 100,000, said composition being a blend of from about 40 to about 90% of a paraffin wax having a melting point from about 140 to about 190 F. and being substantially free of oil, and from about to about 10% of a copolymer of ethylene and a vinyl compound copolymerizable therewith, said percentages being by weight and based on the combined weight of the parafiin wax and copolymer with said copolymer containing from about 60 to about of ethylene and from about 10 to about 40% of the vinyl compound.

2. The tape of claim 1 wherein the vinyl compound is vinyl acetate.

3. The tape of claim 2 wherein the conductive base member is carbon filled paper.

References Cited UNITED STATES PATENTS 3,075,859 1/1963 Relph et a1. 117158 3,189,573 6/ 1965 Oken.

2,886,464 5/1959 Van Dorn 117-76 3,048,553 8/1962 Moss 161-234 3,110,621 11/1963 Doggett et a1. 117-218 3,293,059 12/1966 Stowell 11717.5

ROBERT F. BURNETT, Primary Examiner R. J. ROCHE, Assistant Examiner US. Cl. X.R. 

